1. Field of the Invention
The present invention relates to a semiconductor device with reduced dimensions and a method for fabricating the semiconductor device. The invention relates, for example, to an insulated-gate transistor including a semiconductor gate electrode which is formed of, e.g., polycrystalline silicon (Si). In addition, the invention relates to a semiconductor device with a very fine structure, which includes an array of a great number of basic elements each formed of the above-mentioned insulated-gate transistor, and to a method for fabricating the semiconductor device.
2. Description of the Related Art
In a MOS integrated circuit, for instance, insulated-gate transistors are employed as basic elements. In a semiconductor device in which such basic elements are integrated at high density on a semiconductor chip, a parasitic resistance and a short-channel effect in each insulated-gate transistor increase as the basic elements become smaller in size. It is thus important to form the source/drain region of the insulated-gate transistor so that the source/drain region may have a low resistance and be shallow. In order to lower the resistance of an impurity diffusion region, it is necessary to sufficiently activate the impurity elements that are doped in the impurity diffusion region by, e.g., ion implantation. On the other hand, the impurity elements are diffused in the semiconductor region by annealing for activation (hereinafter referred to as an “activation annealing process”). It is thus necessary to perform the activation annealing process at high temperatures and at high speed.
As a method for instantaneously supplying thermal energy necessary for activating impurity elements, an optical annealing technique has been studied which employs a flashtube in which a rare gas such as xenon (Xe) is sealed, or a laser. Since the light source, which is used in the optical annealing technique, can emit light with a pulse width on the order of milliseconds, it is possible to activate the doped impurity ions with little diffusion. In the optical annealing technique, by increasing the irradiation light energy, the sheet resistance of the impurity diffusion layer can greatly be reduced and the defects due to ion implantation can be remedied. As a result, an enhancement in driving power of the insulated-gate transistor can be expected.
However, in the optical annealing technique, since high thermal energy is supplied instantaneously, heat tends not to escape from the polycrystalline Si on the gate insulation film and the temperature of the polycrystalline Si tends to increase. In particular, the following problem arises with an n-channel type insulated-gate transistor such as an nMOSFET. That is, in the activation annealing process using the optical annealing technique, the withstand voltage of the gate insulation film deteriorates before sufficient thermal energy is supplied to a single-crystal Si substrate. As a result, gate leakage current of the nMOSFET increases.
In order to solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 2004-63574, for instance, discloses a method in which the dosage of impurities, which are ion-implanted in the polycrystalline Si layer, is reduced, or the kind of impurities is changed. According to this method, however, a capacitance forms due to depletion in a bottom part of the gate electrode that is formed of polycrystalline Si. As a result, the driving performance of the insulated-gate transistor deteriorates.
In short, there is a trade-off between the increase in impurity density (high concentration activation) in the source/drain region and gate electrode of the insulated-gate transistor, which includes the gate electrode formed of polycrystalline Si, and the suppression of the gate leakage current. Hence, in the activation annealing process using the conventional optical annealing technique, it has been difficult to secure a process window relating to the heat treatment step in the fabrication of the semiconductor device with the very fine structure, for which a shallow junction is required.